The present invention relates to a process for pretreatment of an aluminum wheel prior to paint.
1. Prior Art
Anodic oxidation (Alumite process), metal coloring, and chemical conversion treatment (reactive chromate process, MBV process, boehmite process, etc.) are well known as processes for surface treatment of aluminum and aluminum alloys. In aluminum coating, reactive chromate treatment, non-chromate treatment (organometallic salt film, etc.), and film treatment by anodic oxidation are carried out as a pretreatment prior to the paint coating to improve the corrosion resistance and paint adhesiveness.
In case of a pretreatment for clear coating utilizing the brightness of aluminum wheels, films by non-chromate treatment (organometallic salt film) are not satisfactory in the corrosion resistance, and films by anodic oxidation have a good corrosion resistance, but its coating is so thick that the brightness is lost. That is, these treatments are not satisfactory as pretreatments for the clear finishing making highly use of the brightness of a base metal. Films by reactive chromate treatment have a good corrosion resistance, but when a colorless film appearance is required, the chromium coating amount (chromium amount in the coating) is limited to not more than 10 mg/m.sup.2, and thus a filiform corrosion resistance is not satisfactory.
As described above, no such pretreatment processes that can give satisfactory corrosion resistance and adhesiveness after paint coating, while maintaining the brightness of aluminum wheels as a base material have been found yet.